Fiber optic attenuators are devices which reduce (attenuate) the strength of a fiber optic signal within a fiber optic network. This may occur because of a mismatch between the transmitters/receivers (transceivers, media converters), or because the media converters are designed for a much longer distance than for which they are being used. The dB ratings are a measure of signal strength and can sometimes be confusing. The chart below will give you an idea of the percent of attenuation of your signal for specific dB values.

Attenuation % (signal reduction) for different dB values
1dB = 21% / 2dB = 37% / 3dB = 50% / 4dB = 60%
5dB = 68% / 6dB = 75% / 7dB = 80%
10dB = 90% / 13dB = 95%  / 15dB = 96.8%